Clinical experience demonstrates that patients who undergo marrow transplantation from unrelated donors have a higher incidence of acute GVHD and rejection than do genotypically matched sibling transplants. Matching for the alleles of the class II genes decreases the risk of acute GVHD and improves survival. The class I genes, HLA-A, B and C, are known to be highly polymorphic and the role of matching for class I alleles is just coming into focus. We have observed a correlation between donor-recipient mismatching for HLA-C genes and increased risk of both graft rejection and cute GVHD. In our analysis of patients who experienced graft rejection, the donor and recipient mismatched HLA-C molecules encoded drastic amino acid substitutions at positions 9,99 and 156. These positions define peptide binding pockets B, D and E and are thought to influence the nature of the P2 and P3 residues of the bound peptide. Based on these clinical findings and together with available peptide data from other investigators, we hypothesize that graft rejection occurred as a result of recognition of highly dissimilar peptides by alloreactive T cells. This project will define the rules that govern peptide binding to HLA-A, B and C molecules (Specific Aim 1) and the biologically relevant peptides recognized by host-derived alloreactive cytotoxic T cell clones (Specific Aim 2). We will test the hypothesis that the peptide repertoire influences the cytotoxic T cell responses as measured by CTL-p to determine whether donor-recipient "compatibility" may be more accurately defined on the basis of biologically relevant peptide repertoires (Specific Aim 3). With the definition of the class I peptide motifs we will identify the genes that encode proteins giving rise to these peptides (Specific Aim 4). This Project will elucidate the basic immunogenetic mechanisms of peptide/MHC interactions and its influence on the alloimmune response. This information will provide the information needed to correlate alloimmune responses and MHC/peptide diversity with clinical outcome following marrow transplantation. The model proposed in this project will serve as a paradigm for the study of induction of tolerance and graft-versus- leukemia effects following transplantation from mismatched donors, and provide a biologic rationale for redefining donor suitability based on peptide/MHC motifs.